The present invention relates to a star sensor for measuring an azimuth and an elevation angle of a fixed star observed from a satellite so as to detect an attitude of the satellite, which is subjected to a spinning motion.
A satellite spins to stabilize its attitude. However, a slight change in mass of the satellite and other disturbances can cause unnecessary rotation and deviation of a spinning axis from a predetermined direction. As a result, a positional error of the satellite's attitude occurs. In order to prevent this, an azimuth and an elevation angle of a fixed star observed from a satellite are detected and the fixed star is identified. These detected values are compared with those obtained when the satellite is located at a predetermined attitude. As a result, the attitude of the satellite is detected.
A conventional star sensor is shown in FIG. 1. Slit plate 4 is arranged on a focal plane of optical system 2 and has two slits A1 and A2 formed in a V shape. Three slits constituting an N shape may be formed in slit plate 4 in place of V slits. Photomultiplier 6 is arranged as a photosensor on an optical axis of optical system 2 behind slit plate 4. An output from image intensifier tube 6 is supplied to signal processing circuit 8. A satellite spins about axis 10 in a direction indicated by arrow .omega., axis 12 of the optical system is perpendicular to spinning axis 10, slit plate 4 is perpendicular to optical axis 12 of the optical system, and a direction along which slit A1 extends is parallel to spinning axis 10.
In this conventional star sensor, star image 14 formed by optical system 2 is moved by the spinning motion spinning of the satellite, as shown in FIG. 2A and sequentially crosses slits A1 and A2. At this time, a light beam from the star passing through slit plate 4 is detected by image intensifier tube 6, and a pulse having amplitude Is corresponding to a light intensity is supplied to signal processing circuit 8. As described above, since slit plate 4 has V-shaped slits A1 and A2, two pulses corresponding to slits A1 and A2 are generated for one star image 14, as shown in FIG. 2B. Time interval T between the pulses is determined by a distance between slits A1 and A2. An incident angle (i.e., an elevation angle of a fixed star) is determined whether star image 14 crosses the upper portion (a small distance between slits A1 and A2) or crosses the lower portion (a large distance between slits A1 and A2). Timings for generating these pulses are determined by a phase angle within one spinning cycle (360.degree.) of the satellite, i.e., an azimuth of the fixed star. Therefore, the pulse generation timings and pulse time interval T are detected by signal processing circuit 8, thereby detecting the azimuth and the elevation angle of the fixed star and hence identifying the fixed star.
However, since image intensifier tube 6 is an electron tube, a high-voltage power source of 1 to 2 kV is undesirably required. In addition, the power source requires high power consumption. The electron tube is large and heavy and has poor reliability. The electron tube poses many problems when it is mounted in a satellite.
A solid-state star sensor has been developed wherein an Si photodiode is used as a photosensor in place of an image intensifier so as to provide a compact, lightweight sensor having low power consumption. An example of such a star sensor is shown in FIG. 3. In this star sensor, two linear Si photodiodes 16 and 18 are arranged in a V shape, and a preamplifier 20 is connected to the output of the photodiodes 16 and 18.
However, the Si photodiode has poor sensitivity as compared with the image intensifier tube and is adversely affected by noise. It is therefore difficult to detect a star darker than a star of the second magnitude or less.